1. Field of the Invention
The present invention relates to a magnetic field sensor having giant magnetoresistive effect elements which cause significant changes in the resistance in response to changes in an external magnetic field, a method for manufacturing the same and an apparatus therefor.
2. Prior Art
Conventionally, as a magnetic field sensor using giant magnetoresistive effect elements, there has been known a magnetic field sensor constructed by bridge-connecting four giant magnetoresistive effect elements as disclosed in Japanese Published Unexamined Patent Application No. Hei 8-226960.
The magnetic field sensor A disclosed in this publication is, as shown in FIG. 12, constructed such that there are provided giant magnetoresistive effect elements 1, 2, 3 and 4 arranged to be apart from one another; the giant magnetoresistive effect elements 1 and 2 are connected through a conductor 5; the giant magnetoresistive effect elements 1 and 3 are connected through a conductor 6; the giant magnetoresistive effect elements 3 and 4 are connected through a conductor 7; the giant magnetoresistive effect elements 2 and 4 are connected through a conductor 8; an input terminal 10 is provided by connecting it to the conductor 6; an input terminal 11 is provided by connecting it to the conductor 8; an output terminal 12 is provided by connecting it to the conductor 5; and an output terminal 13 is provided by connecting it to the conductor 7.
The giant magnetoresistive effect element 1, 2, 3 or 4 has a sandwich structure in which ferromagnetic layers 16 and 17 are provided above and below a non-magnetic layer 15 respectively, and is constructed such that an exchange bias layer 18 such as an antiferromagnetic layer is provided on the one ferromagnetic layer (pinned magnetic layer) 16, and that exchange coupling is achieved by means of this exchange bias layer 18 to pin the direction of magnetization of the ferromagnetic layer 16 in one direction. Also, the direction of magnetization of the other ferromagnetic layer (free magnetic layer) 17 is made freely rotatable in accordance with the direction of the external magnetic field, for example, freely rotatable along the horizontal plane including the ferromagnetic layer 17.
In a magnetic field sensor A having the structure shown in FIG. 12, the direction of magnetization of the pinned magnetic layer 16 in the giant magnetoresistive effect element 1 is made to be frontward as indicated by the arrow 20 in FIG. 13, the direction of magnetization of the pinned magnetic layer 16 in the giant magnetoresistive effect element 2 is made to be backward as indicated by the arrow 21, the direction of magnetization of the pinned magnetic layer 16 in the giant magnetoresistive effect element 3 is made to be backward as indicated by the arrow 23, and the direction of magnetization of the pinned magnetic layer 16 in the giant magnetoresistive effect element 4 is made to be frontward as indicated by the arrow 23. The directions of magnetization of the free magnetic layers 17 in the giant magnetoresistive effect elements 1, 2, 3 and 4 are made to be rightward as indicated by the arrow 24 in FIG. 12 respectively in the absence of an applied external magnetic field.
When an external magnetic field H acts on the magnetic field sensor A shown in FIG. 12, the direction of magnetization 24 of the free magnetic layer 17 rotates by a predetermined angle d as shown in FIG. 13 in, for example, the first and fourth giant magnetoresistive effect elements 1 and 4 so as to meet the external magnetic field H. Therefore, the angular relation with the direction of magnetization 20 of the pinned magnetic layer 16 changes, resulting in a change in resistance. Also, since the direction of magnetization of the pinned magnetic layer 16 in the first and fourth giant magnetoresistive effect elements 1 and 4, and the direction of magnetization of the pinned magnetic layer 16 in the second and third giant magnetoresistive effect elements 2 and 3 are 180.degree. opposite to each other, output having different phases in a resistance change state can be obtained.
In a bridge-connected type magnetic field sensor A shown in FIG. 12, these directions of magnetization are defined as indicated by each arrow. This is because, since when the direction of magnetization of the free magnetic layer 17 changes in response to the external magnetic field H, it is necessary to obtain differential output from the giant magnetoresistive effect elements 1, 2, 3 and 4, the direction of magnetization must be pinned in the antiparallel direction, which is 180.degree. different in direction, between adjacent ones which are adjacent to one another in the giant magnetoresistive effect elements 1, 2, 3 and 4 located left, right, up and down in FIG. 12.
In order to implement the structure shown in FIG. 12, it is necessary to form the giant magnetoresistive effect elements 1, 2, 3 and 4 on a substrate so as to be adjacent to one another, and to fix the directions of magnetization of the pinned magnetic layers 16 in those giant magnetoresistive effect elements adjacent in directions which are 180.degree. different respectively.
In order to control the direction of magnetization of the pinned magnetic layer 16 of this sort, it is necessary to adjust lattice magnetization of the exchange bias layer 18. To this end, it is necessary to apply a magnetic field having a predetermined direction to the exchange bias layer 18 in advance in a state in which it has been heated at a temperature, or higher, called "blocking temperature" at which the ferromagnetism disappears, and to perform heat treatment in which cooling is performed while this magnetic field is being applied.
In the structure shown in FIG. 12, however, since the direction of magnetization of the exchange bias layer 18 must be changed by 180.degree. for each of the giant magnetoresistive effect elements 1, 2, 3 and 4, it becomes necessary to control the direction of the magnetic field for each of the giant magnetoresistive effect elements which have been formed in an adjacent state on the substrate. By means of a method of merely applying a magnetic field by a magnetic field generating apparatus such as an electromagnet from the outside, it is possible to apply the magnetic field only in one direction, and therefore, there has been a problem that it is difficult to manufacture the structure shown in FIG. 12.
For this reason, according to the technique disclosed in the Japanese Published Unexamined Patent Application No. Hei 8-226960, it is described that conductor layers are stacked along the giant magnetoresistive effect elements 1, 2, 3 and 4 formed in an adjacent state on the substrate respectively, and the above-described heat treatment is performed while magnetic fields having different directions are caused to be generated individually from each conductor layer by flowing currents having different directions through each of these conductor layers, whereby the structure shown in FIG. 12 can be implemented. However, Although it is desired to generate a great magnetic field by applying a large current to the conductor film in order to control the lattice magnetization of the exchange bias layer 18, it is difficult to flow a large current through the thin film-shaped conductor film stacked together with the giant magnetoresistive effect elements on the substrate, and in the magnetic field which can be generated from the conductor film, there is a problem that cannot be effectively dealt with by applying a strong magnetic field. Further, since magnetic fields having different directions act on the giant magnetoresistive effect elements 1, 2, 3 and 4 provided in an adjacent state on the substrate from a plurality of conductor films, there has been a problem that it is very difficult to individually cause the strong magnetic field to act on the exchange bias layer 18 in the individual giant magnetoresistive effect elements.